1. Field of the Invention
The present invention relates to a light scanning device used in an image recording apparatus such as a laser printer, a laser facsimile machine, or the like.
2. Description of the Related Art
Conventionally, such light scanning devices are structured such that light modulated by an image signal is deflected and scanned by a light deflector, which is formed by a rotating polygon mirror which rotates at high speed, and is illuminated onto a photosensitive body. As a result, the deflector becomes a source of vibrations, and the optical box or the optical parts which form the light scanning device are excited and vibrate. Imperfections in the image, such as deviation of the image or irregularities in density or the like, arise due to this vibration.
Here, the means described in Japanese Patent Application Laid-Open (JP-A) No. 7-84203 for improving the above-described drawback may be implemented. As illustrated in FIG. 9, in a light scanning device 100 of JP-A-7-84203, a drive mechanism 104, a rotating polygon mirror 106, a lens system 108 and the like are disposed on a plate-shaped body 102. A dynamic vibration absorber 109 formed from a weight 105 and a viscoelastic body 107 is provided at the plate-shaped body 102. (Note that a portion of the structure is simplified in FIG. 9.)
As illustrated in FIG. 3A, a case in which an exciting force of Pcosot is applied to a mass M can be modeled by a system having two degrees of freedom in which an auxiliary vibration system m (mass) and k (spring) are attached to a main vibration system M (mass) and K (spring). K (spring) of the main vibration system corresponds to side plates 112 supporting the plate-shaped body 102.
However, it is generally known that, in a case in which the natural frequency .upsilon.=(k/m).sup.1/2 of the auxiliary vibration system is equal to the frequency .omega. of the exciting force, the constant displacement amplitudes A, a of the masses M, m in the vibration system are A=0, a=-p/k.
However, in the light scanning device 100 of JP-A-7-84203, in order to form the dynamic vibration absorber 109, the weight 105 and the viscoelastic body 107, which are separate parts from the light scanning device 100, must be provided. The number of parts in the entire apparatus increases, and accordingly, the number of operations for assembly increase.
As is clear in FIG. 3B, when the relationship .upsilon.=.omega. collapses, the vibration reducing effect of the dynamic vibration absorber weakens. More specifically, there are unavoidable manufacturing variations of the masses M, m and the springs K, k included in the main vibration system and the auxiliary vibration system. Further, because there are also variations in the frequency .omega. of the exciting force, in actuality, the relationship .upsilon.=.omega. collapses, and vibrations occur in both the main vibration system and the auxiliary vibration system.
In the light scanning device 100 of JP-A-7-84203, the dynamic vibration absorber 109 is provided at a position which is separated in the horizontal direction from the drive mechanism 104 and the rotating polygon mirror 106. Therefore, when vibrations are generated in both the main vibration system and the auxiliary vibration system, as illustrated in FIG. 10, a rotation moment is generated in the entire light scanning device 100 due to the vibration of the weight 105, and a deviation .delta.1 in the scan position of a beam L on a photosensitive body 110 arises. Therefore, the effects which were anticipated are not achieved in actuality, and a drawback arises in that imperfections in the image arise. In particular, when the distance from the light scanning device 100 to the photosensitive body 110 is long, the deviation .delta.1 in the scan position increases.